1. Field
The present disclosure relates to a bleeder circuit, and more particularly to an adaptive bleeder circuit having a bleeder to dynamically compensate a required holding current needed for keeping TRIAC turning on for Alternating Current (TRIAC) dimming control application.
2. Related Art
At night or under insufficient natural light circumstances, artificial illumination may be adopted to provide sufficient illuminance by using a lighting device to project light. However, ordinary artificial illumination usually consumes much power. If the light is dimmed to a darker level (such as 25-50% of the original luminance) under a condition that high illuminance is not required, the energy may be effectively saved. A common dimmer, for example, can be a TRIAC dimmer, an electronic dimmer, or a remote control dimmer (such as an infrared or radio-frequency remote control dimmer).
Generally, the TRIAC dimmer mainly includes a TRIAC used for changing input power of the lighting device. The TRIAC normally maintains a conduction state (on-state) after being activated, and will not be cut off until the current flowing through the TRIAC decreases to be lower than a holding current. When the light is dimmed downward manually, both the input voltage and input current of the lighting device will decrease accordingly. This eventually results in that the current of the TRIAC is less than the holding current such that the TRIAC is cut off unexpectedly, and a flicker problem of the lighting device occurs. In order to alleviate the flicker problem that may occur during dimming, a conventional solution is to add a bleeder or dummy load as the circuit to keep the current flowing through the TRIAC higher than the holding current flowing through TRIAC. By this way, the TRIAC can be kept on successfully at every firing angle without flickering even in the smaller dimming cases.
A common method in the prior art is to connect a constant resistor in parallel with the line input terminals to serve as the bleeder circuit. The extra current extracted from the line input by the bleeder circuit is used for assuring that the line input current is greater than the holding current required for keeping turning on during the normal conduction cycle. However, it should be noted that, since the bleeder circuit uses a constant resistor, the extracted current, relevant to the impedance of the resistor, will vary with the input line voltage. Accordingly, the bleeder circuit cannot dynamically compensate the TRIAC for the exact required current, which will cause much extra power consumed in the bleeder circuit.
In addition, the current extracted by the conventional bleeder circuit is in direct proportion to the input voltage. In other words, since the current extracted by the bleeder circuit is rather limited, when the input voltage is too low, the bleeder circuit cannot provide the TRIAC with the required line current, and the TRIAC will be cut off eventually. When the input voltage is too high, the current extracted by the bleeder circuit also increases accordingly. This will cause extra power being consumed on the bleeder circuit, and result in low working efficiency of the whole line.